The mechanism of calcium channel facilitation in bovine chromaffin cells

Abstract

1. This study was planned to clarify the mechanism of Ca2+ channel facilitation by depolarizing prepulses given to voltage-clamped bovine chromaffin cells. The hypothesis for an autocrine modulation of such channels was tested by studying the effects of a soluble vesicle lysate (SVL) on whole-cell Ba2+ currents (IBa). 2. SVL was prepared from a bovine adrenal medullary homogenate. The ATP content in this concentrated SVL amounted to 3.18 +/- 0.12 mM (n = 4). The concentration of noradrenaline and adrenaline present in the SVL was 11.2 +/- 0.97 and 15.2 +/- 2 mM, respectively (n = 5). A 1:1000 dilution of SVL in the external solution halved the magnitude of IBa and produced a 7-fold slowing of its activation kinetics. The blocking effects of SVL were concentration dependent and quickly reversed upon washout. 3. Inhibition and slowing of the kinetics of IBa by SVL could be partially reversed by strong depolarizing prepulses (+90 mV, 45 ms). This reversal of inhibition, called Ca2+ channel facilitation, persisted in the presence of 3 microM nifedipine. 4. Intracellular dialysis of GDP-beta-S (0.5 mM) or pretreatment of the cells with pertussis toxin (100 ng ml-1 for 18-24 h) prevented the reduction in peak current caused by a 1:100 dilution of SVL; no prepulse facilitation could be observed under these conditions. 5. The receptor blockers naloxone (10 microM) or suramin (100 microM) and PPADS (100 microM) largely antagonized the effects of SVL. Treatment of SVL with alkaline phosphatase or dialysis against a saline buffer to remove low molecular mass materials (< 10 kDa) considerably reduced the activity of SVL. 6. Stopping the flow of the external solution (10 mM Ba2+) gradually reduced the size, and slowed down the activation phase, of the current. Prepulse facilitation of IBa was absent or weak in a superfused cell, but was massive upon flow-stop conditions in the presence or absence of 3 microM nifedipine. 7. Our experiments suggest that facilitation by prepulses of whole-cell current through Ca2+ channels is due to the suppression of an autoinhibitory autocrine loop present in bovine chromaffin cells. By acting at least on purinergic and opiate receptors, the exocytotic release of ATP and opiates will cause a tonic inhibition of the current through a G-protein-mediated mechanism. Such a mechanism will be removed by strong depolarizing prepulses, and will involve preferentially non-L-type channels. In the light of these and other recent results, previously held views on the selective recruitment by prepulses of dihydropyridine-sensitive Ca2+ channels are not tenable.